Controls for the drive of a vehicle with differential speed steering

ABSTRACT

A control drive is provided for the drive of a vehicle, whose steering is effected by a differential driving of the drive wheels of the two sides of the vehicle, where the drive wheels of each vehicle side are driven through a hydrostatic drive unit, whose final control element is connected with a servo piston capable of sliding in an operating cylinder and is acted upon from both sides, in which case the servo piston is acted upon from one side for controlling the travel speed and the control pressure determining the travel speed is reduced for effecting a turn on one side of the vehicle for large turning radii, and for smaller turning radii a pressure is additionally built up on the opposite side of the servo piston, the said pressure being derived from the control pressure determining the travel speed on the other side of the vehicle.

This application is a divisional of my copending application Ser. No.482,142 filed Apr. 5, 1983 now U.S. Pat. No. 4,599,855.

This invention relates to controls for the drive of a vehicle withdifferential speed steering and more particularly to a control for thedrive of a vehicle with differential-speed steering and with ahydrostatic drive unit for driving the drive wheels of each of the twosides of the vehicle.

Such a control is already proposed in German Pat. No. 31 09 491 and isgenerally well perfected and of good design. However, such control stillhas a shortcoming in that, if it is switched to only a slight travelspeed, but the control pressure pickoff is in the extreme out-positionfor turning, the vehicle can effect a travel movement that aninexperienced operator does not immediately anticipate. It is also quitedifficult with this design to achieve an absolute analogy with regard tothe nature and mode of actuation of the actuators between the currentlywidely used vehicles with hydrodynamic drive with turning control byenergy dissipation on the inside of the turn and vehicles with ahydrostatic drive unit on each side of the vehicle.

The present invention proposes to further improve the familiar controlmechanism and to have the vehicle always definitely effect the travelmovements expected even by an inexperienced operator and to have thevehicle equipped with actuators that are analogous in their arrangementand function to those of vehicles with hydrodynamic converters.

This problem is solved in accordance with the invention in that each ofthe two control pressure pickoffs assigned to a turning direction isconnected for pressure supply to the outlet of the control pressurepickoff determining the travel speed and has a pressure reducing valve,which in the nonactuated state allows the control pressure medium toflow through unimpeded and in the actuated state decreases the pressureof the medium as compared with the inflow pressure. It is thus assuredthat the control pressure determining the turning, with which the sideof the servo piston that is not loaded by the control pressuredetermining the absolute travel speed is loaded, cannot be higher thanthe control pressure determining the absolute travel speed, since thelatter is supplied to the control pressure pickoff determining theturning and this pressure pickoff is designed as a valve that can onlyreduce the pressure present, but cannot produce a pressure higher thanthe inflow pressure.

This design facilitates a decrease in the control pressure on the insideof the turn as compared with that on the outside of the turn and onlythen the building up of a counterpressure on the opposite side of theservo piston. Even with the full actuation of the control pressurepickoff influencing one turning direction a control pressure cannot bebuilt up in one operating cylinder, even if at the same time the vehicleis set at only a low travel speed, that adjusts the drive of the vehicleside on the inside of the turn to a higher travel speed than the driveof the side on the outside of the turn, running in the oppositedirection, although it is possible to operate both drives in theopposite direction with the same speed, so that the vehicle turnsprecisely around its vertical axis (i.e., "on a dime").

This design also facilitates additional advantageous refinements. Thetwo pilot valves cannot only be controlled hydraulically, as disclosedin the preferred embodiment or principal application, but selectivelyalso mechanically, electrically or pneumatically.

It is provided in one advantageous embodiment that each of the twocontrol pressure pickoffs assigned to one turning direction consists oftwo pressure reducing valves that are separate with regard to flow, butare coupled together with regard to function, i.e., with respect tocontrol. It can thus be achieved that the control pressure is firstreduced on the inside of the turn and only then is the counterpressurein front of the servo piston built up.

It is provided in one expedient embodiment of the invention that each ofthe two pilot valves is designed as a 4-connection/3-position valve,where each connection of the one connection pair is connected with oneof the two pressure reducing valves of the assigned control pressurepickoff and each of the other two connections of this pilot valve isconnected with a pressure chamber of the operating cylinder of theassigned drive unit. The direction of travel is then selected bycontrolling the two pilot valves.

A further simplification or the posssibility of eliminating one controlpressure pickoff and thus reducing the consturction costs in turnresults from this inasmuch as it is provided in a further expedientimplementation that only one control pressure line is present for thecontrol pressure determining the travel speed, where the controlpressure carried in this control pressure line influences the travelspeed in both directions of travel, i.e., for forward and reversetravel, independently of the direction in which the vehicle istravelling, in which case the direction is selected by switching thepilot valves.

As with the control according to the principal patent application, onlyone arbitrarily actuatable actuator need be provided for controllingboth the direction and the speed of travel. This actuator can be, e.g.,a foot pedal, which, as in normal motor vehicle control, as is alsopresent in the conventional automobile, controls by adjusting the travelspeed, in which case the direction of travel is determined by a shiftlever. Above all, it is also possible however to design this actuator asa hand lever which is installed as is still currently conventional incrawler-type loaders with drive through a hydrodynamic converter. Inthese vehicles known to date it is conventional to provide for steeringwith two pedals, where the pedal located on the inside of the turn isactuated for braking the drive on that side. Precisely this arrangementof the actuators, i.e., the steering pedals, can also be provided in thedesign according to the invention, in which case however each pedal isconnected with a control pressure pickoff such that in construction andmode of actuation there is a complete analogy for the many operators ofthe control arrangements conventional to date, in which case however thedrive can be controlled quite sensitively up to turning on the spot,without dissipation of energy in friction brakes.

The fact that only one control pressure line need be present for thetravel speed also makes it possible to make do with a single controlpressure pickoff for the travel speed. On the other hand, it may beadvantageous to provide one control pressure pickoff for controlling theforward speed and a second control pressure pickoff for controlling thebackward speed, in which case the outlets of these two control pressurepickoffs are connected through a changeover valve that switches as afunction of pressure to the control pressure line carrying the controlpressure that determines the travel speed. Analogously to thearrangement in the principal patent application, this facilitatesreversing the two change-over valves hydraulically, depending on whichof the two control pressure pickoffs delivers the control pressuredetermining the travel speed.

It is provided in an expedient embodiment according to the inventionthat of the two pressure reducing valves located in a control pressurepickoff that is assigned to one turning direction, one is connecteddirectly to the control pressure line carrying the control pressure thatdetermines the travel speed and the other is conencted through achangeover valve that reverses as a function of the pressure and islocated between the two lines leading to a pressure chamber of theoperating cylinder, and the pilot valve and the pressure reducing valveof the drive unit assigned to the other side of the vehicle, to thiscontrol pressure line carrying the control pressure that determines thetravel speed.

In one expedient embodiment the functional coupling of the two pressurereducing valves present in one control pressure pickoff is designed sothat during actuation the pressure reducing valve connected directly tothe control pressure line carrying the control pressure determining thetravel is always actuated first and only then is the other pressurereducing valve connected through the change-over valve and pilot valveto the control mechanism of the other side of the vehicle actuated.Then, as already described, a change-over valve that reverses as afunction of pressure is installed between the two lines emerging fromone connection pair of a pilot valve and leading to a pressure chamberof the operating cylinder; its outlet is connected with the inlet of thepressure reducing valve of the control pressure pickoff assigned to thedrive unit of the other side of the vehicle, which is actuated secondlyin the functional sequence of actuation, in which case the otherpressure reducing valve of this first-mentioned control pressure pickoffis connected directly to the outlet of the control pressure pickoffdetermining the travel speed.

In another implementation form, which is closer to the design accordingto the principal patent application No. P 31 09 491.0 and in which, justas in this principal patent application, two travel speed controlpressure pickoffs assigned to one direction of travel are provided,where a control pressure line departs from each of these two controlpressure pickoffs and they lead respectively to a pressure chamber ofthe two operating cylinders, in which case the one control pressurepickoff is connected to one side and the other control pressure pickoffto the other side of each operating cylinder. Also according to thisimplementation the two connections of a pilot valve that are connectedwith a pilot valve are connected to these lines leading from a controlpressure pickoff to a pressure chamber, and a pressure reducing valve isconnected to the one line leading from a pilot valve to a controlpressure pickoff assigned to turning. This pressure reducing valve shutsoff the line in the nonactuated state and in the actuated state connectsthe line in a throttled manner with a pressureless drain container. Asecond pressure reducing valve is also connected to the control pressureline of the other side of the vehicle and it is also closed in thenon-actuated state, while in the actuated state it connects this controlpressure line of the other side in a throttled manner with the secondchamber of the operating cylinder.

In the foregoing description of my invention, I have set out certainobjects, purposes and advantages of this invention. Other objects,purposes and advantages of this invention will be apparent from aconsideration of the following description and the accompanying drawingsin which the components of the circuit diagrams of controls pertainingto the invention up to the operating cylinders or the componentsdeviating from these circuit diagrams are depicted and wherein:

FIG. 1 shows the circuit diagram of an embodiment with two controlpressure pickoffs for travel speed and direction and one controlpressure line;

FIG. 2 illustrates an embodiment with two control pressure pickoffs fortravel speed and direction and with mechanical connection to the pilotvalves;

FIG. 3 shows an embodiment with only one control pressure pickoff forthe travel speed; and

FIG. 4 shows an embodiment with two control pressure pickoffs for travelspeed and direction and with two control pressure lines for the travelspeed.

FIG. 5 is an embodiment in exactly the same format as that of FIG. 4 butwith a different type of pressure pickoff.

A control pressure line 2 departs from a control pressure pump 1 andleads to two control pressure pickoffs 3 and 4. A return line 5 departsfrom each of these two control pressure pickoffs 3 and 4 and leads to apressureless container. A control pressure line 6 departs from thecontrol pressure pickoff 3 and a control pressure line 7 departs fromthe control pressure pickoff 4 and the two control pressure lines 6 and7 lead to a change-over valve 8 that reverses as a function of thepressure. A control pressure line 9 carrying the control pressure thatdetermines the travel speed departs from this change-over valve 8 andbranches into two control pressure lines 10 and 11.

The two pressure reducing valves 12 and 13 together form the controlpressure pickoff assigned to the left-hand side of the vehicle and areboth actuated by the pedal 14, in which case the mechanical coupling isdesigned so that when the pedal 14 is actuated, the pressure reducingvalve 12 is always actuated first up to its maximum value and only thenis the pressure reducing valve 13 actuated.

In an analogous manner, the two pressure reducing valves 15 and 16together form the control pressure pickoff that is assigned to theright-hand side of the vehicle and can be jointly actuated with the aidof a pedal 17, in which case the pressure reducing valve 15 is alwaysactuated first and only then is the pressure reducing valve 16 actuated.

The two lines 18 and 19 leaving one of the two pressure reducing valves12 and 13 that together form a control pressure pickoff are connected totwo connections of the 4-connection/3-position pilot valve 20. Two lines21 and 22 are connected to the other two connections of this pilot valve20; one of them is connected to a pressure chamber of the operatingcylinder 23 in which the servo piston 24 is capable of sliding and whichis connected with the final control element of the left-hand side of thevehicle (not shown either). The servo piston 24 is loaded by two springs25 in the direction to the neutral position.

The servo piston 34 is connected through a lever (or connecting rod) 124with the final control element of a hydraulic pump 125, which isconnected through the lines 126 and 127 with the hydraulic motor 131,whose final control element is in turn connected with the servo piston132, which is capable of sliding in the operating cylinder 133, which isconnected to the line 38.

Analogously, a servo piston 27 is capable of sliding in the operatingcylinder 26 and it is connected with the final control element of thedrive unit (not shown) of the right-hand side of the vehicle.

The servo piston 27 is connected through a kinematic connection 137 withthe final control element of the hydraulic pump 128, which is connectedthrough the lines 129 and 130 in closed circuit with the hydraulic motor136, whose final control element is connected with the servo piston 134,which in turn is capable of sliding in the operating cylinder 135, whichis connected to the line 34. The line 28 departing from the pressurereducing valve 15 leads to a connection of the pilot valve 29, while theline 30 departing from the pressure reducing valve 16 leads to a secondconnection of the pilot valve 29, in which case two lines 31 and 32 areconnected to the second connection pair of this pilot valve 29, each ofwhich is connected to a pressure chamber of the operating cylinder 26. Achange-over valve 33 that reverses as a function of pressure is locatedbetween the two lines 31 and 32; at its outlet a line 34 is connectedand leads to the operating cylinder of a motor (not shown) of thehydrostatic drive unit of the right-hand side of the vehicle. Restrictor35 that limits a backflow is located between the pilot valve 29 and theconnection of the changeover valve 33 in each of the two lines 31 and32.

A line 36 branches off from this line 34 and leads to the inlet of thepressure reducing valve 13 of the left-hand side of the vehicle.

A change-over valve 37 that reverses as a function of pressure isconnected between the two lines 21 and 22 and a control pressure line 38is connected to its outlet. Line 38 leads to the operating cylinder (notshown) of the hydraulic motor of the drive unit of the left-hand side ofthe vehicle. A line 39 is connected to this line 38 and leads to theinlet of the pressure reducing valve 16 of the control pressure pickoffassigned to the right-hand side of the vehicle. Analogously as on theright-hand side, a restrictor 35 is incorporated in the lines 21 and 22between the pilot valve 20 and the connection of the change-over valve37.

A change-over control pressure line 40 departs from the control pressureline 6 and leads to a change-over control pressure chamber of the pilotvalve 29. A change-over control pressure line 40a branches off from thelatter and also leads to a change-over control pressure chamber of thepilot valve 20. Analogously, a change-over control pressure line 41branches off from the control pressure line 7 and leads to the other twochange-over control pressure chambers of the two pilot valves 20 and 29.

Either the control pressure pickoff 3 or pickoff 4 can be selectivelyactuated by means of an arbitrarily actuatable actuator (3a), in whichcase the vehicle is switched to forward travel when control pressurepickoff 3 is actuated and switches to backward travel when pickoff 4 isactuated, and the degree of actuation determines the travel speed.Turning is controlled such that the right-hand side of the vehicle isthe inside of the turn when pedal 17 is actuated, and the turning iscontrolled such that the left-hand side of the vehicle is the inside ofthe turn when pedal 14 is actuated, independently of the direction oftravel.

The mode of operation is as follows: If control is effected through thesaid actuator (not shown) of the control pressure pickoff 3, the controlpressure line 6 is thus acted upon by pressure, with the result that thechange-over valve 8, which reverses as a function of pressure, connectsthe control pressure line 6 with the control pressure line 9 carryingthe control pressure determining the travel speed, such that the controlpressure determined by the control pressure pickoff 3 is propagatedthrough the control pressure lines 6, 9, and 10 to the pressure reducingvalve 12 and through the lines 6, 9, and 11 to the pressure reducingvalve 15. Because neither pressure reducing valve 12 or 15 is acutated,they allow the pressure to pass through unhindered. The control pressurepresent in the control pressure line 6 is propagated through thereversing pressure line 40 also to the upper reversing pressure chamberof the pilot valve 29 in the drawing and through the reversing pressureline 40a to the upper reversing pressure chamber of the pilot valve 20in the drawing, with the effect that the pilot valve 29 is manipulatedso that it connects the lines 28 with line 31 and lines 30 with line 32and that the pilot valve 20 connects the line 18 with line 21 and andline 19 with line 22. As a result, the control pressure present in line9 is conveyed through the pressure reducing valve 12 and lines 18 and 21into the upper chamber of the operating cylinder 23 in the drawing andon the other hand through the lines 11, 28, and 31 into the upperpressure chamber of the operating cylinder 26 in the drawing. The twoservo pistons 24 and 27 are thus acted upon from the same side andcontrol the drive units of both vehicle sides in the same manner forforward travel. The other two pressure chambers of the operatingcylinders 23 and 26 are relieved of pressure through the unactuatedpressure reducing valves 13 and 16, although the pressure standing inline 31 is present at the inlet at the pressure reducing valve 13through the reversing valve 33 and the line 35 and the pressure standingin the control pressure line 21 is present at the inlet of the pressurereducing valve 16 through the change-over valve 27, the line 38, and theline 39.

If the pedal 17 is now first actuated somewhat for turning, the pressurereducing valve 15 is thus also actuated and thus reduces the controlpressure in lines 28 and 31, so that the upper control pressure chamberof the operating cylinder 26 in the drawing is acted upon by a lowercontrol pressure than the upper control pressure chamber in theoperating cylinder 23 in the drawing, with the result that theright-hand drive unit in the drawing is set to a lower output r.p.m.than the left-hand drive unit in the drawing. This continues until thecontrol pressure in the upper pressure chamber of the operating cylinder26 in the drawing is completely reduced and thus the servo piston 27 andthus the final control element of the drive unit connected with it isbrought into the neutral position, such that the right-hand drive unitis switched to a stoppage of the output shaft and thus a track-layingvehicle equipped with this drive turns around the right-hand track. Nowif the pedal 17 is further actuated, the pressure reducing valve 16 isalso actuated, with the result that it reduces the pressure present inthe control pressure line 39 and conveys it through line 30 and line 32to the lower pressure chamber of the operating cylinder 26 in thedrawing.

The servo piston 27 is thus displaced upward in the drawing from theneutral position, with the result that the drive unit of the right-handside of the vehicle in the drawing is switched to backward travel, i.e.,in the opposite direction from the drive unit of the left-hand side ofthe vehicle in the drawing. If pedal 17 is depressed up to the endposition and thus opens the pressure reducing valve 16 to full passage,the same pressure is present in the line 30 and thus in the lowerpressure chamber of operating cylinder 26 in the drawing as in line 29and thus in line 21, with the result that the drive units of both sidesof the vehicle operate at the same speed, but in opposite directions.This causes the vehicle to turn around its vertical axis. Since thein-feed pressure at the pressure reducing valve 16 cannot be higher thanthe pressure in line 21 as a result of the connection of line 39 throughthe change-over valve 37 to the line 21 even if the control pressure inline 21 is very low, the control pressure in the lower pressure chamberof then operating cylinder 26 in the drawing cannot be greater than thecontrol pressure in the upper pressure chamber of the operating cylinder23 in the drawing, such that an undesirable travel in the oppositedirection can never occur.

Accordingly, the same effect results if pedal 14 is acutated instead ofpedal 17.

If the control pressure pickoff 4 is actuated instead of pickoff 3, thecontrol pressure line 7 carries the control pressure, which propagatesthrough line 41 to the lower reversing pressure chambers of the pilotvalves 20, 29 in the drawing, so that accordingly the same effectoccurs, but only with respect to the other pressure chambers of theoperating cylinders 23 and 26.

The actuating lever 45, with which the direction and speed of travel canbe arbitrarily regulated, is shown in FIG. 2. In the neutral position ofthe actuating lever 45 shown in the drawing neither the control pressurepickoff 3 or pickoff 4 are actuated and the pilot valves 20a and 29a arein their neutral position in which they connect the two pressurechambers in the operating cylinder 23 with each other and with thepressure reducing valve 13, and the two pressure chambers in operatingcylinder 25 with each other and with the pressure reducing valve 16. Ifthe actuating lever 45 is moved clockwise in the drawing, i.e., in thedirection of the white arrow, the control pressure pickoff 3 is actedupon and the pilot valves 29a and 20a are actuated. The pilot valve 20adiffers from pilot valve 20 and the pilot valve 29a differs from pilotvalve 29 only in that they are not actuated hydraulically, butmechanically, and pilot valve 20a also differs from pilot valve 20 inthat the arrangement of the switching positions is interchanged, sincein the arrangement according to FIG. 1 both pilot valves 20 and 29 areactuated in the same direction, while in the design according to FIG. 2both pilot valves are actuated in the opposite direction due to themechanical connection.

The actuating lever 45 is supported in a housing-hinged support 44 andrigidly connected with the two-arm lever 46, which is connected with theslides of the two pilot valves 20a and 29a and is arranged so that whenthe actuating element 45 is moved clockwise, i.e., in the direction ofthe right-hand arrow, not only are the pilot valves 29a and 20regulated, but the control pressure pickoff 3 is at the same timeactuated to a degree dependent on the extent of lever movement. Elasticbridging elements 47 are provided in the connection rods to the slidesof the pilot valves 20a and 29a so that the latter can reach their endposition before actuation of the control pressure pickoff 3 begins.

If the actuating lever 45 is actuated in the counterclockwise direction,i.e., in the direction of the black arrow, the pilot valves 20a and 29aare regulated in the opposite direction and the control pressure pickoff4 is actuated.

The implementation according to FIG. 3 differs from that according toFIG. 2 in that only one control presure pickoff 43 is provided insteadof the two control pressure pickoffs 3 and 4 assigned to one directionof travel; this pickoff 43 is also connected to the delivery line 2 ofthe control pressure auxiliary pump 1. Because only this one controlpressure pickoff 43 is present, the control pressure lines 6 and 7 andthe valve 8, which reverses as a function of pressure, drop out, suchthat the control pressure pickoff 43 is connected directly to thecontrol pressure line 9, which carries the control pressure thatdetermines the travel speed.

A movement transfer element 49 is supported in a housing-solidlongitudinal guideway that permits only a translatory displacement,precisely up and down vertically in FIG. 3. This element 49 has two armsthat lie opposite the two-armed lever 46. If the actuating element 45 ismoved clockwise, i.e., in the direction of the white arrow, theright-hand arm of the two-arm lever 46 lies against the right-hand armof the movement transfer element 49 and presses it down to the extentthat the actuating lever 45 is moved clockwise. If the actuating lever45 is moved in the opposite direction, i.e., in the direction of theblack arrow, the left-hand arm of the two-arm lever 46 in the FIG. 3 ispressed against the left-hand arm of the movement transfer element 49.In both cases, independently of the direction of movement of theactuating lever 45, the result of moving it is a depressing of themovement transfer element 49, depending on the degree of movment of theactuating lever 45, in which case the control pressure pickoff 43 isactuated by the movement transfer element 49 to a degree correspondingto the movement of the actuating lever 45.

Two control pressure pickoffs 3 and 4 are again provided in theimplementation according to FIG. 4, where the control pressure line 6departing from the control pressure pickoff 3 branches into two controlpressure lines 56a and 56b, of which line 56b is connected to the lowerpressure chamber of the operating cylinder 23 in the drawing and line56a is connected to the lower pressure chamber of operating cylinder 26in the drawing. Analogously, the control pressure line 7 departing fromthe control pressure pickoff 4 branches into two control pressure lines57a and 57b, of which line 57b is connected to the upper pressurechamber of the operating cylinder 23 in the drawing and line 57a isconnected to the upper pressure chamber of operating cylinder 26 in thedrawing.

The line 61 joins a connection of the pilot valve 20 with the line 57band line 62 joins the second connection of this connection pair of thepilot valve 20 with line 56b. Analogously, one connection of the pilotvalve 29 is connected through line 63 with line 57a and the secondconnection of the same connection pair of the pilot valve 29 isconnected through line 64 with the line 56a. Of the other twoconnections of the pilot valve 20 one is connected through line 18 witha pressure reducing valve 52, which in the non-actuated state shuts offline 18 and in the actuated state connects it in a throttled manner witha pressureless tank 5. The second connection of this connection pair ofthe pilot valve 20 is connected through line 19 with the outlet of apressure reducing valve 53, whose inlet is connected through line 59with the line 28. The control pressure pickoff assigned to the left-handside of the vehicle and capable of being actuated by the pedal 14 thusconsists of the two pressure reducing valves 52 and 53.

A vehicle is of course normally constructed so that the same controlmembers are located on both sides. In the implementation according toFIG. 4, therefore, the control pressure pickoffs 52, 53 actuatable bythe pedal 14 on the left-hand side of the vehicle with other elementsare identical to the control pressure pickoffs 52, 53 actuatable by thepedal 17 and assigned to the right-hand side of the vehicle. It is thuspossible to design the switching arrangement with one control pressurepickoff 52, 53 on the two vehicle sides, as it is shown in FIG. 4,however, it is also possible to equip a control on both sides withcontrol pressure pickoffs 54 and 55 on the two vehicle sides asillustrated in FIG. 5.

In FIG. 5, a restrictor is designated by 54, 55; it shuts off the linecompletely in the nonactuated state and increasingly opens it in theactuated state. The function of the restrictors 54 and 55 thuscorresponds basically to the functions of the pressure reducing valves52 and 53.

The mode of operation is as follows: If the control pressure pickoff 3is actuated, the lower pressure chamber of operating cylinders 23 and 26is acted upon through the lines 6, 56a, and 56b, with the result thatthe servo pistons 24 and 27 are displaced upward in the drawing to thesame degree. At the same time, line 62 is connected with line 18 throughthe pilot valve 20 and line 64 is connected with line 28 through thepilot valve 29. Now if the steering pedal 14 is actuated, the pressurein line 18 and thus the pressure in line 62 and thus in the lowerpressure chamber of the operating cylinder 23 is reduced, in which casethe pressure medium is prevented by restrictors 35 from flowing back. Ifthe pedal 14 is further depressed, the pressure reducing valve 53 beginsto open and thus conveys pressure medium from line 28 through the line59 into line 19 and thus into the upper pressure chamber of theoperating cylinder 23.

Accordingly, the same effect results in the opposite direction if pedal17 is actuated or if control pressure pickoff 4 is actuated instead ofpickoff 3.

The operation of the embodiment of FIG. 5 operates precisely asdescribed above in connection with FIG. 4, except the pickoffs 54 and 55are restrictors which shut off the lines 18 and 19 and 28 and 30completely in the non-actuated state and open them increasingly in theactuated state as described above, thus corresponding basically inoperation to the pressure reducing valves 52 and 53 of FIG. 4.

The foregoing specification I have set out certain preferred practicesand embodiments of this invention, however, the invention may beotherwise embodied within the scope of the following claims.

I claim:
 1. In a control for the drive of a vehicle havingdifferential-speed steering and having a hydrostatic drive unit on eachside of the vehicle for operating the drive wheels and a final controlelement for each of the two drive units connected with a spring-loadedservo piston capable of sliding in an operating cylinder, theimprovement comprising a pressure chamber on each side of the servopiston in the operating cylinder connected with each drive unit, meansproviding an arbitrarily adjustable control pressure on each servopiston, first control pressure pickoff means for controlling the forwardand reverse travel connected with operating cylinders of the drive unitsof both sides of the vehicle, two additional second control pressurepickoffs, each assigned to one turning direction connected with theoperating cylinder of each drive unit of the vehicle, each servo pistonbeing capable of being acted upon on one side by the first controlpressure pickoff means for controlling the direction and speed oftravel, and, on the other side, may be acted upon by a second controlpressure pickoff of the pair of pickoffs assigned to turning travel forcontrolling the turning radius, a pilot valve located in each connectionline between a second control pressure pickoff assigned to turningtravel and the operating cylinder of the corresponding drive unit, anarbitrarily actuable direction and speed selection lever operativelyconnected to said first pressure pickoff means, means for switching eachsaid pilot valve as a function of the position of the arbitrarilyactuable direction and speed selective lever, said pilot valveconnecting the control pressure from the first pressure pickoff throughthe second control pressure pickoff assigned to turning travel with apressure chamber of an operating cylinder of the drive unit, anarbitrarily actuable turn selection lever operatively connected to eachsecond control pressure pickoff, means in each said second pickoff forcontrolling the control pressure through said pilot valve as a functionof the position of the arbitrarily actuable turn selection lever torestrict the connection from the first pressure pickoff to the operatingcylinder on the drive on the inside of the curve, whereby the otherpressure chamber of said operating cylinder is acted upon by the controlpressure from the first control pressure pickoff means that determinesat least one of the speed and the direction of travel, said secondcontrol pressure pickoff assigned to one turning direction of thecontrol pressure pickoff pair determining the direction and radius ofturning reducing the control pressure in the said one chamber of theoperating cylinder on the inside of the turn while the second controlpickoff on the opposite side connected to the operating cylinder on theopposite side is unchanged and acted upon by the control pressurepickoff that determines the direction and speed of travel, each saidsecond pickoff including means for first reducing the control pressureon said one pressure chamber of said operating cylinder and thensequentially connecting said control pressure to the other pressurechamber of the operating cylinder, each pilot valve being a4-connection/3-position valve, where a pressure reducing valve of one ofthe second control pressure pickoffs assigned to one turning directionis connected to each connection of the one pair of connections of apilot valve and each of the two other connections of this pilot valve isconnected to a pressure chamber of the operating cylinder of theassigned drive unit, and wherein only one control pressure line ispresent for the control pressure determining the speed of travel andthat the direction of travel is controlled by arbitrary actuation of thepilot valves.
 2. A control according to claim 1, wherein a singlearbitrarily actuatable actuator, preferably a hand lever, is providedfor adjusting the direction and speed of travel.
 3. A control accordingto claims 1 or 2, wherein two control pressure pickoffs, each assignedto one direction of travel, are provided for the speed of travel, inwhich case a change-over valve which switches as a function of pressureis connected to the outlets of the two control pressure pickoffs, theoutlet of this valve being connected to the single control pressure linefor controlling the speed of travel.
 4. A control according to claim 1,wherein only one control pressure pickoff is provided for controllingthe travel speed.
 5. A control according to claim 1, wherein the pilotvalves are connected mechanically with the actuator.